Currently, radio links employ several frequency bands on VHF (30 . . . 300 MHz), UHF (300 MHz . . . 3 GHz), SHE (3 . . . 30 GHz), and EHF (30 . . . 300 GHz) bands. Ever higher frequencies have been used because mobile services have almost entirely used the lower frequency bands (below 3 GHz). Presently, many radio link systems operate in the 38 GHz frequency range, which, at least initially, is the range for the antenna according to the present invention. As the principle of the antenna is not in any way tied to frequency, the antenna design of the invention is intended for use in the micro and millimeter ranges.
Radiation characteristics required of radio link antennas are specified in international standards. For example, the ETSI (European Telecommunications Standards Institute) standard prETS 300 197 specifies the highest levels permitted to side lobe levels in the radiation pattern of a 38 GHz radio link antenna. Thus, the starting point of designing radio link antennas is typically such that the antenna gain must be higher than a specific minimum level, but also such that the side lobe levels remain lower than specific limits. The gain cannot, therefore, be increased indefinitely because it would increase the side lobe levels accordingly.
Requirements set for radio link antennas are strict, and, on frequencies presently used, the radiation characteristics specified in the standards have successfully been fulfilled only with different kinds of horn plus lens or reflector antennas (parabolic antennas).
Apart from adequate radiation characteristics, antenna manufacturers and especially antenna users (customers) desire physically small antennas. Particularly when the terminal point of the radio link is at the customer's site, it is important for the antenna to blend into the background as well as possible (i.e., fit into a small space).
Laws of physics largely determine the antenna cross sectional area. In other words, the antenna must have a specific capture area or its aperture must have specific dimensions. Instead, through structural design, dimensions of the antenna in the thickness direction can be modified. For example, the drawback of the aforementioned horn plus lens or reflector antennas is that these antennas cannot be made compact due to their operating principle. In the aforementioned 38 GHz range, for example, such antennas are at least on the order of 20 cm thick.
Small dimensions in the thickness direction can be obtained by planar antennas (a planar antenna refers to a design in which the feeders and reflector elements of the antenna are very close to one another in the thickness direction). Planar antenna designs are often based on microstrip technique, which results in an insufficient gain due to the high loss of the microstrip structure. Many planar antenna designs also share the drawback of being narrow-band (required characteristics are only obtained on a narrow frequency band). Some planar antennas also have the disadvantage of being unsuitable for mass production due to the very strict dimensioning requirements on the higher frequencies used today. Antenna manufacturers desire an antenna design that can be mass produced.